Mobile communication networks deploy various techniques for measuring the geographical locations of communication terminals. Such techniques are used, for example, for various surveillance applications and for providing Location Based Services (LBS) and emergency services in cellular networks. (In the description that follows, the term “locating users” is used, for the sake of brevity, to mean “locating the communication terminals used by the users.” Communication terminals whose locations are tracked are referred to as target terminals or targets.)
Some location tracking techniques, referred to as network-based techniques, are carried out by the base stations and other network-side components, without using dedicated location tracking hardware or software at the mobile terminal side. For example, a technique called Uplink Time Difference of Arrival (U-TDOA) determines the user position by calculating the difference in time required for a user transmission to reach different base station sites. The arrival time measurements are made by Location Measurement Units (LMUs) installed at selected base station sites. Another network-based technique, referred to as Angle of Arrival (AOA), determines the user position by establishing lines of bearing from base station sites to the user.
Other location tracking techniques are terminal-based, i.e., use dedicated hardware or software in the mobile terminal. For example, some techniques use measurements performed by a Global Positioning System (GPS) receiver installed in the communication terminal. In Assisted GPS (A-GPS) techniques, the GPS measurements are assisted by an assistance server external to the mobile terminal. The assistance server is sometimes equipped with another GPS receiver, whose position is known a-priori. Another terminal-based technique is Enhanced Observed Time Difference (E-OTD), in which the terminal measures the time differences between signal arrivals from different base stations. A similar terminal-based technique is called Enhanced Forward Link Trilateration (EFLT).